In digital electronics, a ring oscillator with odd number of stages of inverter (NOT Gates) is generally used as a source of clock. However, non-crystal oscillators, like the ring oscillators, show huge frequency variations due to variations in device parameters, particularly, temperature, voltage and process. These frequency variations can be as high as 100% and therefore, compensation circuitry is needed to achieve high frequency stability.
In the past, numerous compensation circuits have been suggested for compensation due to variations in process, voltage and temperature (combined PVT) to achieve frequency stability of a ring oscillator. Even though these compensation circuits are able to reduce the frequency variations, they are not able to provide very high frequency stability, at a wide range of operating voltages and temperatures, while consuming very low power. Particularly, conventional compensation circuits in the past do not achieve very high frequency stability, say in the range of less than ±2% frequency variation while operating at voltage range of 1.6 V to 3.6 V, temperature range of 40° C. to 125° C. and consume low power (current of less than 20 μA across entire operating voltage range of 1.6 V to 3.6 V, i.e. in the range of 32 μW to 72 μW).
A publication “An Ultra-Low-Power Oscillator with Temperature and Compensation for UHF RFID Transponder” (Published in: RADIOENGINEERING, VOL. 22, NO. 2, June 2013, wang et al), discloses a low power oscillator with temperature and process compensation. While the circuit uses hybrid resistor string for frequency stability, it does not achieve high frequency stability. Further, operating voltage and temperature range is limited.
U.S. Pat. No. 7,586,380 and EP1523806 disclose circuits for temperature and process compensation for a ring oscillator. However, frequency variation for these oscillators upon compensation is still very wide. Further, these oscillators with compensation circuits consume significant amount of power and therefore are not suitable for low power applications.
Another problem with the compensation circuits known in the art is that their design is not flexible. Particularly, the compensation circuits described in the prior arts are not able perform at different frequencies and are also not portable to various CMOS bulk technologies.
Therefore, there still remains an unfulfilled need of implementing a ring oscillator with compensation for PVT variations at low power consumption. Also, there is a need for compensation circuit design which is highly scalable for generating any stable frequency and easily portable to any CMOS bulk technologies.
In view of the foregoing, there exists a need for an on-chip ring oscillator which has less than 2% frequency variation, over wide supply voltage range (1.6V to 3.6V), temperature range (−40 C to 125 C) and process variation without using any external (off-chip) component (resistor or capacitor) in any bulk CMOS technology. Also, the aim of the invention is to achieve this less than 2% frequency variation, preferably less than 2%, at very low power consumption and scalability to any stable frequency.